A gas diffusion electrode (GDE) consumes or is depolarized by a gas feed while allowing direct electronic transfer between the solid and gas phase. Together with the electrolyte, the GDE provides a path for ionic transfer, which is just as critical. GDEs are typically constructed from a conductive support, such as a metal mesh, carbon cloth, or carbon paper. This support is often called a web. The web is coated with hydrophobic wet-proofing layers, and finally, a catalytic layer is applied most commonly to one face. While the catalytic layer can consist of very fine particles of a precious metal mixed with a binder, many employ the methods similar to that in Petrow, et al., U.S. Pat. No. 4,082,699. This patent teaches the use of using finely divided carbon particles such as carbon black as the substrate for small (tens of angstroms) particles of the nobel metal. Thus called a xe2x80x9csupportedxe2x80x9d catalyst, this methodology has shown superior performance and utilization of the catalyst in electrochemical applications. However, the application of this supported catalyst as well as wet proofing layers to the web engages the need for a well-dispersed mix.
Often, GDEs are cited as key components in Fuel Cells. Here, the anode is typically depolarized with hydrogen while the cathode is depolarized with oxygen or air. The resulting products are energy in the form of electricity, some heat, and water. Examples of acid or alkaline fuel cells are well known. However, some have also realized that the energy-producing quality of a fuel cell can be adapted to industrial electrochemical processes and thus save energy and hence reduce operating costs.
GDEs also may allow the creation of a commodity directly from a gaseous feedstock. For example, Foller, et al. (The Fifth International Forum on Electrolysis in the Chemical Industry, Nov. 10-14, 1991, Fort Lauderdale, Fla., Sponsored by the Electrosynthesis Co., Inc.) describe the use of a GDE to create a 5 wt. % hydrogen peroxide in caustic. In this case, oxygen is the feedstock and a specific carbon black (without noble metals) is the feedstock and a specific carbon black (without noble metals) is the catalyst. A typical chlor-alkali cell uses two solid electrodes to produce sodium hydroxide and chlorine. In this case, both the anode and cathode expend energy to evolve gas, and special measures are taken to keep the resulting hydrogen away from the chlorine due to a potentially explosive mixture. The typical chlor-alkali cathode can be replaced with an oxygen-depolarized cathode, as has been shown by Miles et al. in U.S. Pat. No. 4,578,159 and others. A cell run in such a manner saves approximately one volt, and the hydrogen/chlorine problem is eliminated. Aqueous hydrochloric acid is an abundant chemical by-product. One can recover the high-value chlorine by oxidizing solutions of HCl, and thus recycle the chlorine as a feedstock to the chemical plant.
Electrolysis becomes extremely attractive when the standard hydrogen-evolving cathode is substituted with an oxygen-consuming gas diffusion electrode due to the significant drop in energy consumption. The ability of the gas diffusion electrode to operate successfully in this and the preceding examples is acutely dependent on the structure of the gas diffusion electrode: for in all these cases, the electrode serves as a zone for liquid-gas-solid contact, as a current distributor, and most importantly, as a liquid barrier. The use of solid polymer electrolytes has greatly expanded the field of electrochemistry. As summarized above, electrochemical processes depend on the transfer of ionic and electronic charge through the use of an anode, cathode, and an ionic liquid electrolyte. However, with the advent of the solid polymer electrolyte fuel cell, the traditional liquid phase has been replaced with a membrane composed of a polymer electrolyte that transfers ionic charge under typical electrolytic conditions. One can deposit a catalyst layer directly on the membrane, or attach a gas diffusion electrode to one or both faces of the conducting membrane. Such an assembly can be called a membrane electrode assembly (MEA), or for fuel cell applications, a PEMFC (proton exchange membrane fuel cell).
These solid polymer electrolytes are often composed of ion-conducting membranes that are commercially available. For example, in addition to the previously mentioned Nafion (a cation exchange membrane), Asahi Chemical and Asahi Glass make perfluorinated cation exchange membranes whereby the ion exchange group(s) are carboxylic acid/sulfonic acid of carboxylic acid. These companies produce cation exchange membranes with only the immobilized sulfonic acid group as well. Non-perfluorinated ion exchange membranes are available through Raipore (Hauppauge, N.Y.) and other distributors such as The Electrosynthesis Co., Inc. (Lancaster, N.Y.). Anion exchange membranes typically employ a quaternary amine on a polymeric support and are commercially available as well.
Nafion is typically employed in some fuel cells. For the hydrogen/air (O2) fuel cell, hydrogen and oxygen are fed directly to the anode and cathode respectively, and electricity is generated. For these xe2x80x9cgas breathingxe2x80x9d electrodes to perform, the gas diffusion electrode structure must be highly porous to allow three phase contact between the solid electrode, the gaseous reactant, and the liquid or near liquid electrolyte. In addition to providing a zone for three-phase contact, the gas diffusion electrode structure aids in making electrical contact to the catalyst, enhances transport of reactant gasses into the zone, and provides for facile transport of product away from the zone (e.g. water vapor).
In addition to a gaseous hydrogen fuel and gaseous air (O2) oxidant, others employ a mixed phase system such as the methanol/air(O2) fuel cell. Here, liquid methanol is oxidized at the anode while oxygen is reduced at the cathode. Another utilization for ion-conducting membranes and gas diffusion electrodes includes the electrochemical generation of pure gasses (for example see Fujita et al. in Journal of Applied Electrochemistry. vol. 16, page 935, (1986), electro-organic systhesis [for example see Fedkiw et al. in Journal of the Electrochemical Society, vol. 137, no. 5, page 1451 (1990)], or as transducers in gas sensors [for example see Mayo et al. in Analytical Chimica Acta, vol. 310, page 139, (1995)].
Typically, these electrode/ion-conducting membrane systems are constructed by forcing the electrode against the ion conducting membrane. U.S. Pat. No. 4,272,353, No. 3,134,697; and No. 4,364,813 all disclose mechanical methods of holding electrodes against the conducting membrane. However, the effectiveness of a mechanical method for intimately contacting the electrode to the polymer membrane electrolyte may be limited since the conducting membrane can frequently change dimensions with alterations in hydration and temperature. Swelling or shrinking can alter the degree of mechanical contact.
Thus, an alternative method of contacting the electrodes with the polymer membrane electrolyte involves direct deposition of a thin electrode onto one or both sides of the conducting polymer substrate. Nagel et al. in U.S. Pat. No. 4,326,930 disclose a method for electrochemically depositing platinum onto Nafion. Others have employed chemical methods whereby a metal salt is reduced within the polymer membrane [for example see Fedkiw et al. in Journal of the Electrochemical Society, vol. 139, no. 1, page 15 (1192)].
In both the chemical and electrochemical methods, one essentially precipitates the metal onto the ion conducting membrane. This precipitation can be difficult to control due to the nature of the ion-conducting polymer membrane, the form of the metal salt, and the specific method employed to precipitate the metal. As the goal of a thin, porous, and uniform metal layer is often not met via precipitation, practitioners have turned to other deposition methods. For example, ion beam assisted deposition techniques are disclosed in co-pending provisional patent application by Allen et al. (Ser. No. 60/035,999); a method for coating the membrane with an ink composed of the supported catalyst and solvent is disclosed by Wilson and Gottesfeld in the Journal of the Electrochemical Society, volume 139, page L28, 1992; and a method of using a decal to deposit a thin layer of catalyst onto the ion-conducting membrane is summarized by Wilson et al. in Electrochimica Acta, volume 40, page 355, 1995. Thus, these approaches differ from the previous strategy by the catalyst layer being deposited onto the ion conducting membrane, and a gas diffusion structure is subsequently placed against this catalyst layer.
Regardless of whether the catalyst is fixed to the membrane, or coated onto an uncatalyzed gas diffusion electrode and then bonded to the membrane via mechenical, and/or thermal means, the structure and composition of the component contacting the catalyst contributes to the overall MEA performance. This component is variously called a xe2x80x9cdiffuserxe2x80x9d, an electrode xe2x80x9cbackingxe2x80x9d, xe2x80x9cgas diffusion mediaxe2x80x9d, a xe2x80x9cgas diffusion layerxe2x80x9d, or an xe2x80x9cuncatalyzed gas diffusion electrodexe2x80x9d and can predominate MEA performance during operation at high current density. We will use the term diffuser to encompass all these synomyms. A diffuser is a material that: 1) provides electrical contact between the catalyst and the electrochemical cell current collector, 2) distributes and facilitates efficient transport of feed gas or gasses to the electrode, and 3) becomes a conduit for rapid transport of product(s) from the electrode. Thus the electrode is the catalytic layer or zone characterized by a three-phase interface of solid, liquid, and gas whereas the diffuser is a two-phase structure for gaseous (or liquid) transport and electrical contact.
There are a few commercial providers for diffusers. Gore Associates (Elkton, Md.) offer Carbel(trademark), a conductive, microporous polymer. E-TEK, Inc. (Natick, Mass.) offers uncatalyzed versions of the gas diffusion electrodes found in their catalog. Of these, the uncatalyzed ELAT(trademark) is listed as the best material for MEA applications. The gas diffusion electrode structure designed for providing a three-phase zone, current contact, and a liquid barrier is being adopted for MEA applications.
Typical ELAT construction is detailed in U.S. Pat. No. 4,293,396 by Allen et al. Here, a carbon cloth serves as the web. Carbon black is prepared for application to the carbon web by using techniques listed in U.S. Pat. No. 4,166,143 whereby solutions of Vulcan XC-72 or Shawinigan acetylene black (SAB) are mixed with water, ultrasonically dispersed with a sonic horn, mixed with Teflon(copyright) (TFE), and filtered. Layers of SAB mix serve as the wetproofing layer on each side of the web. Finally, layers of (catalyzed) Vulcan mix are coated onto one side of the assembly. Although the importance of mix penetration into the web is discussed, the actual coating method is not disclosed. The reported products were of limited lot size, so may have been individually prepared. After the final coat, the assembly may be sintered in air at a temperature sufficient to cause the Teflon to flow, typically 300-350xc2x0 C. This double sided structure was designed with the intent to create an electrode that both achieves good gas distribution and contact with the catalyst while providing a hydrophobic barrier to prevent electrolyte transport completely through the electrode. Regardless, no information is relayed as to how this structure could be produced with economical means.
Similarly, a typical ink application is described by Ralph et. al. in the Journal of the Electrochemical Society, Vol. 144, page 3845, 1997 and references therein. Here, the goal is to minimize platinum usage. A gas diffusion electrode is constructed by using silk screen technology to coat a carbon paper web. The ink is comprised of catalyzed carbon black and binders including Teflon. The authors claim a resulting electrode structure comparable to that described by wilson and Gottesfeld or Wilson et al. cited previously above. If GDEs and ion conducting membranes are to be used in large volume, commercial processes such as power generation in electric vehicles, then one must meet a significant reduction in component cost. Thus, while the authors endorse the need for inexpensive manufacturing processes, they describe a batch coating design, which inherently limits product throughput.
In both the ink and mix preparation methods, it is generally accepted that the ultrasonic horn serves an important role in dispersing the carbon in solvent. Since the carbons are high surface area substances, it is important to prepare a uniform and stable suspension. Carbon blacks do not xe2x80x9cwet-outxe2x80x9d without a significant input of energy or shear into the solution. Some also modify the solution with additives as well to induce high shear. The ultrasonic horn performs this function of wetting-out by way of high frequency electrical energy directed from a stainless steal tip immersed in the solution. The action of the horn generates pressure waves through the vessel and produces high shear through cavitation. Although suitable for limited production runs or RandD sized samples, there are several limitations to ultrasound. First, since the energy is projected from a single source, i.e., the horn, the power is a function of the distance from the horn, and will diminish significantly as one moves away. Second, as the action of the carbon black on the horn leads to abrasion and accelerated corrosion, the projected power spectrum emanating from the horn changes in time. For these reasons, ultrasound may not be appropriate for production of large quantities of diffusers.
With the rise of PEMFCs as suitable clean power sources, and the parallel increase in the use of MEAs in industrial and sensor applications, there is a need for a diffuser tailored for these materials. The current diffuser technology employs structures that were originally designed for liquid electrolyte systems. In addition, the current routine use of the sonic horn produces carbon black dispersions for coating that may be non-uniform and difficult to control for production of large batches of diffuser. Furthermore, the current manufacturing methodology is limited in its applicability to continuously coating a webxe2x80x94a step believed to be crucial in producing an inexpensive product.
It is an object of this invention to provide improved diffuser structures with transport properties for MEA type electrodes.
It is a further object of this invention to introduce a method of manufacture that is compatible with continuous automation.
It is a still further object of the invention to introduce a dispersion methodology that provides an unexpected increase in performance from diffusers and gas diffusion electrodes fabricated from carbon blacks preparing using this technique.
These and other objects and advantages of the invention will become obvious from the following detailed description.
The novel gas diffusion electrode of the invention comprises an electrically conductive web provided on at least one side with a wet-proofing layer of a suitable polymer provided with an electrocatalyst thereon. The electrically conductive web is preferably a carbon cloth web or carbon paper or a metal mesh. The wet-proofing layer may also contain a dispersion of carbon black such as SAB.
The construction of the standard ELAT grew out of many refinements, geared to producing a general-purpose gas diffusion electrode that would work under numerous electrolytic conditions. Lindstrom et al. (U.S. Pat. No. 4,248,682) and the previously cited U.S. Pat. No. 4,293,396 document the progress of the ELAT development. The final structure of the ELAT electrode is determined by the underlying support web, the quantity and kind of carbon black coated onto the web, and the quantity of binder (often Teflon) mixed with the carbon black.
Typically, a final layer of liquid Nafion or ionomer is applied to the face or front of the GDE diffuser to aid in making contact to the electrode (MEA). Such solutions are readily available and come as a 5-10% wt ionomer with an equivalent weight of 1100 or less. Typical levels of Teflon in the Vulcan mix are 5-80% by weight, more preferably 30-70% by weight. The total weight of solid varies by electrode type, but ranges from 0.5 to 25 mg/cm2. The weight of solids is determined by the number of coats applied to the web, and obviously, the weight delivered per pass by the coating device. While any number of the conducting carbon blacks can be employed, for example Shawinigan Acetylene Black, Vulcan XC-72, Black Pearls 2000, or Ketjen Black, in general, the carbon black selected for wet-proofing is hydrophobic while the carbon black selected as the catalyst or electrode layer is more hydrophilic. The Nafion ionomer coated on the face can vary from 0.1 to 2 mg/cm2 is preferred. FIG. 1 is a schematic to delineate these various layers that comprise the structure of the ELAT gas diffusion electrode.
We have changed the structure of the ELAT to accomodate the different reactant and product transport and electronic contact requirements of MEAs. FIG. 1 also shows a comparison of the standard ELAT structure to two embodiments of a new gas diffuser structure. In comparing diffuser type xe2x80x9cAxe2x80x9d of FIG. 1 to the standard ELAT, one notes both a reduction in the number of coated layers, which translates to less total deposited solids, and with coating layers being placed on only one side of the carbon cloth web. The uncoated side of the web is now oriented toward the gas feedstream while the coated layers are placed against the electrode i.e, the face of the membrane electrode assembly). As will be shown in the Examples, these reduced layers and single-sided coatings allow for a reduced number of fabrication steps, and a thinner, more open structure amendable to high gas flux rates.
For diffuser type xe2x80x9cAxe2x80x9d, there are still two or more types of carbon black employed in the architecture of the structure. These are selected so as to create a gradient of hydrophobicity throughout the structure, as well as to provide a layer than can be more easily wetted at the catalyst interface. However, there are applications where a single kind of carbon black is appropriate, and diffuser type xe2x80x9cBxe2x80x9d in FIG. 1 illustrates this alternative structure. For diffuser type xe2x80x9cBxe2x80x9d, one or more coats of carbon black and binder are applied on one side of the web. This diffuser would be oriented as type xe2x80x9cAxe2x80x9d, that is, the uncoated side is towards the feedgas plenum while the coated side is against the electrode face of the MEA. Diffuser type xe2x80x9cBxe2x80x9d is easier to fabricate, and is the least expensive to manufacture.
While much focus has been made on the structure and performance of gas diffusion electrodes, little contribution has been made in the nature and effect of carbon black preparation methods for gas diffusion electrodes. While the sonic horn is frequently cited, we show here surprising enhancements in diffuser and gas diffusion electrode performance through other dispersion methods. For example, one preferred method introduces a pressurized flowing stream of solvent and carbon black in a xe2x80x9cYxe2x80x9d shaped chamber that divides the flow into two streams, which are recombined downstream using another xe2x80x9cYxe2x80x9d. The effect of splitting and recombining the stream introduces high shear and pressure differences on the solvent and carbon black, and effectively wets out the particles in a uniform and consistent manner. A commercial device is available through such companies as Microfluidics (Newton, Mass.). Other methodologies use rotor/stator methodology whereby one set of blades is fixed while the other set is spun at high rates around the fixed set. Such action creates high shear on the sample. Rotor/stator operation are often performed in batch mode. Another device is a mill where a spinning barrel with plates performs the function of delivering shear energy to the solution. Kady Company (Scarborough, Me.) provides a range of these machines. These and similar devices are called xe2x80x9chomogenizersxe2x80x9d and perform the vital function of dispersing solids into solvent in a uniform and consistent manner. The following Example section describes such a preparation and reports results for diffusers and gas diffusion electrodes unanticipated by simple homogenization of the carbon black solution.
While the placement and number of carbon black layers can control structure, and the method used to disperse the carbon black also determines performance, the technique employed to coat a web with mix determines the final structure as well. The previously cited ELAT patents describe a successful coating on the carbon cloth web results from physically penetrating into the woven structure to encase the fiber bundles with mix. Thus, the coating methodology most appropriate for this function is slot-die, knife-over-blade, or spraying followed by a knife operation. Slot-die coating is the preferred method as the slot acts as a control mechanism that meters out a fixed amount of mix. The weight of solids placed on the web is determined by the line speed, pump rate through the slot die, and mix composition (% solids). Furthermore, since the slot-die acts through creating a constant mass of mix between the slot-die head and the moving web, this coating action serves to both give some penetration into the cloth and compensate for surface roughness inherent in the cloth.
While slot-die has been used to coat various solid and porous substrates, using the slot-die to create gas diffusion electrodes and diffusers is a novel application. Typical widths of a slot-die range from 5-250 mm, but larger dies can be constructed. The gap of the slot die can be controlled via shims, but a typical range is between 4 and 100 mils, and more preferably 15-30 mils. Both the coating of the mix and the size of the drying sections of the coating machine determine the line speed, as the freshly coated web is next run into a heated chamber. Typical line speeds range from 0.1 to 5 m/min. Multiple coats can be applied by a series of slot-die stations, or re-running a freshly-coated web through the machine. Other attachments to a manufacturing line would include a continuous sintering oven and a slitting machine to cut the final product into the desired dimensions.
For mixes consisting of carbon black or catalyzed carbon black and Teflon, a Gravure style coating method can be employed as well. Gravure coating employs a spinning rod that is dipped in mix at the lower half and then contacted with the moving web at the other upper segment. Typically the gravure-coating head spins in a direction opposite the direction of the moving web, allowing some penetration of the mix into the web. The quantity of the mix applied to the web per pass is controlled by the mix rheology, line speed, gravure rotation speed and gravure imprint pattern, and the area of the web contacting the head. Gravure coating works best with low viscosity mixes.
The selection of a coating method as slot-die, gravure, knife-over-plate, or spraying is dependent on the fluid dynamics of the mix, mix stability during the coating process, and the electrode and/or diffuser structure desired on the web. One is not limited to one coating method. Typically, more than one coating station can be applied to the moving web to build up a multi-layer structure if so desired, whereupon the selection of coating station is dependent on the requirements of the mix.
In some cases, the composition of the dispersed carbon black mix is modified by adding additives such as iso-propyl alcohol (from 0.1 to 100%, more commonly between 5 to 30%, and preferably 25%), Fluorinert FC 75 or similar, Neoflon Ad-2CR, polyvinyl alcohol, polyox, or similar stabilizers.
In some operations it is preferable to avoid iso-propyl alcohol, for example due to the constraints and costs of handling organic vapors, and a water-based mix is employed. For this type of mix, one of more of the following stabilizers and thickeners could be employed: Fluorinert FC 75 or similar; Neoflon Ad-2CR; polyvinyl alcohol, ethylene glycol, polyethylene glycol alkyl ether; Polyox(copyright); Triton(copyright) X100; Tween(copyright); Joncryl 61J, Rhoplex AC-61, Acrysol GS (acrylic polymer solutions); and naphthalene formaldehyde condensate sulfonates.
The electrocatalyst may be any of those conventionally used such as platinum or a rhodiumxe2x80x94rhodium oxide catalyst described in U.S. patent application Ser. No. 013,080 filed Jul. 26, 1998. The specific coating method and stabilizer is dependent on the structure of diffuser desired.
In the following examples, there are described several preferred embodiments to illustrate the invention. However, it should be understood that the invention is not intended to be limited to the specific embodiments.